The hydrogenation of unsaturated polymers is well known in the prior art. Usually a solution of polymer in an inert solvent is contacted at elevated temperature with hydrogen under pressure in the presence of a heavy metal catalyst, which is often a transition metal catalyst such as nickel, cobalt, iron, platinum and the like. Nickel is a particularly useful and efficient hydrogenation catalyst, especially in the form of nickel compounds activated by special reduction methods.
Unsaturated polymers are hydrogenated or otherwise treated for a variety of reasons, frequently using a nickel catalyst. The presence of olefinic double bonds in the polymers makes them susceptible to oxygen attack and to deterioration by actinic radiation; saturation of olefinic double bonds greatly improves environmental stability. Hydrogenation may improve color. Polyethylene has been produced by hydrogenation of elastomeric polybutadiene (Ind. and Eng. Chem. 45, 1117-22 (1953), and Rubber Chem. and Tech. 35, 1052 (1962)). In certain block co-polymers or homopolymers resistance to flow under stress when hot is improved by hydrogenating the aromatic rings to alicyclic rings.
A common problem shared by all of these types of hydrogenated polymers is the deleterious effect of catalyst residues remaining after hydrogenation. The quantity of metal residues to be removed may be as high a 5% w although substantially smaller amounts usually are present. Nickel, for example, discolors the product and may cause polymer deterioration by promoting reactions with air and actinic radiation; it must therefore be removed almost completely. Filtration may be carried out first to remove much of the catalyst residue but residual contamination is very difficult to remove by purely physical separation; chemical reaction and separation are required. Furthermore, especially with highly viscous polymer solutions filtration often is an impractical means of removing even gross amounts of catalyst residue.
One typical method utilized in the art to remove catalyst residue is the utilization of a dilute solution of sulfuric acid. In this process the dilute sulfuric acid is contacted with the polymer which has been dissolved in a suitable solvent. After contact the polymer-containing phase is separated from the aqueous phase and then the polymer-containing phase is contacted with live steam. The live steam drives off the polymer solvent and converts the polymer into polymer crumb. The main drawback to this process is that a residual amount of sulfuric acid because of its high boiling point, will remain on the polymer crumb and will constitute an impurity that can have a deleterious effect on the finished polymer. For example, the residual sulfuric acid can serve as a catalyst to degrade the polymer, sulfate residually unsaturated bonds, induce crosslinking reactions, etc. The instant process on the other hand provides a means whereby the amount of residual acid in the polymer crumb can be readily reduced by the application of heat and/or vacuum.